The ionization chamber of an ion source functions under vacuum and requires the material that is to be ionized to be fed in gaseous form with great accuracy and reproducibility.
Many manufacturing processes also are conducted in vacuum. Those which incorporate chemical reactions with a workpiece typically require introduction of reagents in gaseous form, the reagents being made to react with one another and/or the workpiece through specific process chemistries. Such processes may result in altering the composition of the work piece, deposition of thin films on the work piece, or etch or removal of material from the workpiece. In semiconductor manufacturing, for instance, such processes must be performed with great accuracy and reproducibility.
Thus, for ion sources as well as for work piece processing chambers, the introduction of a precise and stable flow of gas into a vacuum chamber is required. While many feed materials are available in gaseous form from pressurized gas cylinders, others are only available in solid form. Solid materials require special handling steps different from those used with gaseous sources. Among the solid materials of interest are decaborane, octadecaborane, indiumtrichloride, trimethylindium and triethylantimony.
Solids of interest typically have low vapor pressure and must first be sublimated through heating in a reduced pressure environment to produce a volume of vapor. This vapor must then be introduced into the vacuum chamber at the flow, or number of molecules per second, required by the operation to be conducted in the chamber. Since this flow requirement is similar to that required for the introduction of normal gases, standard gas handling equipment has been used for delivery of solid-derived vapor, but with mixed success. In typical gas handling, the gas source is held at a pressure P0 substantially higher than the inlet delivery pressure, PD, for the vacuum chamber. In order to accurately control the flow of gas into the vacuum chamber, PD must be accurately controlled. This is usually accomplished by a commercially available mass flow controller (MFC) located between the gas source and vacuum chamber inlet. An MFC is a digitally controlled device which varies its conductance to match the delivered mass flow (in grams per second) with the requested mass flow, in a closed-loop manner. Since MFC's are commonly used with relatively high pressure gas sources, MFC's are commonly constructed to operate in a range of correspondingly small conductances, which establish relatively large pressure drops. For vaporized solid materials such as the borohydride decaborane (B10H14) or octadecaborane (B18H22), this approach suffers from several serious problems.
The vapor pressure of such solid borohydrides is low, so the material must be heated close to its melting point (100 C for decaborane) to establish a sufficiently elevated vapor pressure to permit use of the MFC. This risks decomposition of the borohydride molecule which is thermally sensitive.
Since the borohydride vapor readily condenses on surfaces, especially surfaces below the temperature at which the material was vaporized, clogging of relatively small MFC conductances (small passages) results in unstable operation and early component failure.
These problems have largely stood in the way of commercially viable implementation of vapor flow control systems for the controlled delivery of such borohydride vapor feed to ion sources, in which the produced ion beam is used in an ion implanter for the doping of semiconductors.
Further complications ensue when the vapors are derived from a fixed solid charge. Typically, to provide a large surface area, the material of the charge is placed in the vaporizer in powder form. The vaporizing area of the fixed charge diminishes over time as the charge is consumed, and especially when the solid materials are susceptible to molecular disassociation if temperatures become too high. Serious problems arise especially when the operation in which the vapors are to be employed requires accurate maintenance of vapor flow, which is often the case.
The control of flow of vapors from solid materials has not been as accurate as desired, and has involved the necessity of frequent maintenance of equipment, for instance to disassemble flow control equipment to remove deposits of condensed material that affect their operation. All of these detrimental conditions confront the ion implantation of semiconductor substrates when seeking to use the desired dopant materials decaborane, octadecaborane and other thermally unstable or otherwise thermally sensitive compounds.